In dual energy imaging, two images of the same object are acquired under different x-ray beam conditions, such as beam energy and filtration. For example, high- and low-energy images of the same object can be acquired. These images can then be decomposed to produce material specific images, such as soft-tissue and bone-only images.
Double-shot acquisition, which requires a time lapse between acquisition of the high- and low-energy images, promotes detective quantum efficiency and provides an improved detectability index for dual energy imaging as compared to sandwiched detectors. The use of double-shot acquisition, however, introduces the potential for misregistration between the high- and low-energy images. In particular, misregistration occurs as a result of anatomical motion during the time between the two images. Misregistration between the high- and low-energy images may result in artifacts in a combined image.
Misregistration as a result of anatomical motion may occur at various scales. For example, misregistration may occur as a result of bulk anatomical motion of the patient between acquisitions. Misregistration may also occur on a smaller scale as a result of cardiac or respiratory motion. Finally, misregistration may occur on an even smaller scale such as motion associated with fine pulmonary vasculature. Therefore, there is a need to provide a projection image registration method operating at various levels of scale and resolution to resolve misregistration errors prior to dual energy decomposition.